Solvent extraction is used in industrial operations as a method to recover valuable metals. The means of implementing this technology is the availability of suitable metal extractants. These extractants are typically organic soluble complexes that allow the transfer of the metal values from an aqueous to the organic solution containing the extractant when the appropriate aqueous and organic solutions are brought into contact. In the course of this transfer, unwanted impurities, both metallic and non-metallic, depending upon the extractant and conditions employed, are left behind in the aqueous phase, thus separating them from the metal values. To recover these metal values from the organic solution, an aqueous stripping solution may be used. The nature and conditions used to strip the metal from the organic is dictated by the organic extractant and metal values of interest and should result in the purification and concentration of the metal values. The organic extractant is regenerated and recycled at the conclusion of the stripping process. The extraction/stripping process can be represented generally as follows: EQU MA.sub.AQ +E.sub.ORG .fwdarw.ME.sub.ORG +A.sub.AQ Extraction Cycle EQU ME.sub.ORG +S.sub.AQ .fwdarw.MS.sub.AQ +E.sub.ORG Stripping Cycle EQU M=Metal EQU A.sub.AQ =Aqueous solution originally containing metal values EQU E.sub.ORG =Extractant in organic solution EQU S.sub.AQ =Aqueous stripping solution
It can be seen from this representation that the metal values move from the aqueous phase to the organic phase in the extraction cycle, then move back to the aqueous phase from the organic phase in the strip cycle. The metal values maya require further processing by conventional methods for complete recovery.
Solvent extraction processes for recovering metal values as described above are known. See, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Vol. 6, pp. 850-851, Vol. 9, pp. 713-714. Also, U.S. Pat. No. 3,224,873 issued, Dec. 21, 1965 to R. R. Swanson, discloses a solvent extraction process employing certain oxime extractants for the recovery of copper.
In particular, U.S. Pat. No. 3,971,843, issued Jul. 27, 1976 to J. Helgorsky et al., discloses a solvent extraction process employing certain substituted hydroxyquinolines for the recovery of gallium from alkaline aqueous solutions.
Recently, U.S. Pat. No. 4,741,887 issued May 3, 1988 to J. Coleman, et al., discloses a solvent extraction process employing certain N-organohydroxamic acids as extractants of gallium from acidic and alkaline aqueous solutions.
The recovery of gallium from sodium hydroxide solutions by solvent extraction using alkylated 8-hydroxyquinoline as the extractant was described by Sato and Oishi in Hydrometallurgy, 16, 315-324 (1989).
Judd and Arbuch, SME Publication #90-147 (1990), discuss the solvent extraction of gallium from sulfuric acid solutions using octyl phenyl acid phosphate (OPAP).
Gallium is a highly valued metal, with use in the electronic, photovoltaic, and laser industries, as well as having other technical uses. Aqueous solutions containing gallium metal values can be obtained from various sources, including zinc production process streams, alumina production process streams, and from the acid leaching of elemental phosphorous production flue gas residues, also know as "treater dust".
Other metal ions may be present in a gallium solution, often in large excess with respect to the gallium ion, depending on the source of the aqueous solutions. These other ions may include aluminum, zinc, iron, cadmium, potassium, and phosphorous. A successful commercial extractant must be capable of selectively extracting gallium in the presence of these competing ions. In addition, once the gallium is extracted onto the organic solution, it should easily be stripped into another aqueous solution in a concentrated form for further processing.